Atherosclerosis is among the leading causes of death in developed countries. Some of the independent risk factors associated with atherosclerosis include the presence of relatively high levels of serum LDL cholesterol and relatively low levels of serum HDL cholesterol in affected patients. As such, some anti-atherosclerotic therapy regimens include the administration of agents (e.g., statins) to reduce elevated serum LDL cholesterol levels.
Agents that increase patient HDL cholesterol levels can also be useful in anti-atherosclerotic therapy regimens. HDL cholesterol is believed to play a major role in the transport of cholesterol from peripheral tissues to the liver for metabolism and excretion (this process is sometimes referred to as “reverse cholesterol transport”). ABCA1 is a transporter gene involved in HDL production and reverse cholesterol transport. Upregulation of ABCA1 can therefore result in increased reverse cholesterol transport as well as inhibition of cholesterol absorption in the gut. In addition, HDL is also believed to inhibit the oxidation of LDL cholesterol, reduce the inflammatory response of endothelial cells, inhibit the coagulation pathway, and promote the availability of nitric oxide.
Liver X receptors (LXRs), originally identified in the liver as orphan receptors, are members of the nuclear hormone receptor super family and are believed to be involved in the regulation of cholesterol and lipid metabolism. LXRs are ligand-activated transcription factors and bind to DNA as obligate heterodimers with retinoid X receptors. While LXRα is generally found in tissues such as liver, kidney, adipose tissue, intestine and macrophages, LXRβ displays a ubiquitous tissue distribution pattern. Activation of LXRs by oxysterols (endogenous ligands) in macrophages results in the expression of several genes involved in lipid metabolism and reverse cholesterol transport including the aforementioned ABCA1; ABCG1; and ApoE.
Studies have been conducted in LXRα knock-out (k/o), LXRβ k/o and double k/o mice to determine the physiological role of LXRs in lipid homeostasis and atherosclerosis. The data from these studies suggested that in double k/o mice on normal chow diet, increased cholesterol accumulation was observed in macrophages (foam cells) of the spleen, lung and arterial wall. The increased cholesterol accumulation was believed to be associated with the presence of reduced serum HDL cholesterol and increased LDL cholesterol, even though the total cholesterol levels in the mice were about normal. While LXRα k/o mice did not appear to show significant changes in hepatic gene expression, LXRβ k/o mice showed 58% decrease in hepatic ABCA1 expression and 208% increase in SREBP1c expression suggesting that LXRβ may be involved in the regulation of liver SREBP1c expression.
Data obtained from studies employing two different atherosclerotic mouse models (ApoE k/o and LDLR k/o) suggest that agonists of LXRα or β can be relatively effective in upregulating ABCA1 expression in macrophages. For example, inhibition of atherosclerotic lesions could be observed when ApoE k/o and LDLR k/o mice were treated with LXRα or β agonists for 12 weeks. The tested agonists were observed to have variable effects on serum cholesterol and lipoprotein levels and appeared to cause a relatively significant increase in serum HDL cholesterol and triglyceride levels. These in vivo data were found to be consistent with in vitro data obtained for the same agonists in macrophages.
In addition to the lipid and triglyceride effects described above, it is also believed that activation of LXRs results in the inhibition of inflammation and proinflammatory gene expression. This hypothesis is based on data obtained from studies employing three different models of inflammation (LPS-induced sepsis, acute contact dermatitis of the ear and chronic atherosclerotic inflammation of the artery wall). These data suggest that LXR modulators can mediate both the removal of cholesterol from the macrophages and the inhibition of vascular inflammation.